The present invention relates to thermal switches and more particularly to an automatic thermal switch having a pivoting thermal transfer plate.
In many areas of modern technology, such as cryogenic refrigeration, spacecraft, and electronics, it is desirable to control the flow of heat from one area or object to another. For example, in space vehicles it is often necessary to absorb or reject heat from the environment to achieve a desired thermal control over temperature sensitive items (e.g. semiconductor circuitry) in a payload. A direct and fixed heat path between the environment and items is often not acceptable because when the spacecraft field of view is full sun, the amount of heat flow to the items can be so great that the items are destroyed. Similarly, when the spacecraft field of view is deep space, the loss of heat, by black body radiation, from the items can be so large that the items become inoperative.
Two common prior art devices used for thermal control in spacecraft are louvers or an opening and closing cover plate (lid). The exposed surface of either the louvers or the cover plate has, for example, a low absorptivity, high emissivity surface coating which rejects incoming radiation from the sun and enhances the heat radiated from the surface, effectively preventing overheating of a heat-producing, temperature sensitive payload item. The louvers or lid are opened to expose a radiating plate having a low emissivity to space when the spacecraft field of view is deep space. In this way, the internally produced heat can be rejected, but in a controlled manner, such that the payload item does not get excessively cold. Both the lid and louver system are analogous to providing a surface with variable absorption and emission properties.
The chief disadvantage of the lid or louver systems is that they are susceptible to physical damage because they must be affixed to an exterior spacecraft surface. Another disadvantage lies in the magnitude of mechanical motion involved with opening and closing the louvers or lid which produces unwanted force reactions and tends to increase the probability of improper or incomplete functioning. A third disadvantage is that there is still some radiative heat loss from the louvered system when exposed to deep space which necessitates the use of on-board heaters to maintain equipment at operating temperatures. Finally, the need for lids or louvers which cover the entire radiating surface area often results in a system which is heavier than an equivalent number of thermal switches.
Thermal switches have been proposed to vary the thermal conductivity or rate of heat transfer between one area or object and another. One such thermal switch is in contact with a cold plate and includes a sealed extendible bellows containing a flexible wick and a thermally conductive plate carried by the bellows. Freon is encased within the bellows which, along with the flexible wick, acts as a heat pipe. Heat from the cold plate is transmitted to the Freon which evaporates and causes the bellows to expand. The expansion of the bellows causes the conductive plate to contact a hot plate so that heat is transferred between the plates.
Another type of thermal switch includes a thermal actuator in contact with a sensor block. A piston extends from the actuator into a cup in a heat sink. The piston is biased by a pair of springs to keep the piston from contacting the heat sink during normal operation. Heat transferred to the actuator from the sensor block causes the piston to extend into the cup and contact the heat sink while compressing the springs. Thus, heat can flow between the heat sink and sensor block.
Another prior art type of thermal switch variably conducts heat between a heat source plate and a heat sink plate. A plurality of bellows are attached to the heat source plate through conductive spacers. The bellows contain an expandable fluid and carry a thermally conductive plate which is disposed between the two plates. As the heat source temperature rises, the fluid within the bellows expands to cause the thermally conductive plate to contact the heat sink plate so that heat is transferred between the plates.
However, such prior art thermal switches have proved to be unreliable in operation in high vibration environments such as spacecraft, and uneconomical to produce in any large quantities because of their complexity. In addition, such thermal switches have heretofore been of the normally "open" type (low thermal conductivity), with a heat path being provided between a pair of plates only when it is desired to have a "closed" (high thermal conductivity) heat path.
Accordingly, an object of the invention is to provide a new automatic thermal switch that is simple and reliable.
It is further object of this invention to provide an automatic thermal switch that produces relatively small force reactions.
It is still a further object of this invention to provide an automatic thermal switch that effectively controls heat dissipation.
It is an additional object of the invention to provide an automatic thermal switch that is relatively lightweight.
It is another object of this invention to provide an automatic thermal switch which is substantially unaffected by vibration.
It is still an additional object of the invention to provide an automatic thermal switch which can be arranged to provide a low thermal conductive heat path or a high thermal conductive heat path.
It is a further object of this invention to provide a plurality of thermal switches that selectively provide a desired amount of thermal conductivity between two areas.
It is yet another object of the invention to provide automatic thermal switches that control heat flow between a pressurized equipment compartment and the outer skin of a spacecraft.